1. Department of Physics, Federal University of Pelotas, P.O. Box 354, 96010-900 Pelotas, RS, Brazil
2. Department of Science and Technology, Federal University of São Paulo, 12231-280 São José dos Campos, SP, Brazil
3. São Carlos Institute of Chemistry, University of São Paulo, P.O. Box 780, 13560-970 São Carlos, SP, Brazil